1. Field of the Invention
The invention relates generally to a method and apparatus for removal of impurities from inert gases, such as noble gases and nitrogen. In particular, the invention relates to a two-stage purification process conducted at room temperature which achieves removal of contaminants to less then one part per billion (ppb). The first stage contains nickel on a substrate, and the second stage contains a getter alloy.
2. Brief Description of the Prior Art
Numerous methods are disclosed in the prior art for removal of impurities from inert gases. Many methods use a getter material, elevated to high temperatures, for removal of the impurities. In co-pending U.S. application Ser. No. 08/505,136, filed Jul. 21, 1995 (published as WIPO Publication No. WO 97/0345), there is disclosed a method for removal of impurities from noble gases and nitrogen using three temperature zones in a single purifier vessel. In the first zone of the vessel, the gas is preheated to a temperature greater than 400.degree. C.; in the second zone, the preheated gas is contacted with a getter material at greater than 335.degree. C. for removal of impurities such as methane, water, carbon monoxide, nitrogen, oxygen, and carbon dioxide; in the final zone, the gas being purified is contacted with a second getter material at a temperature greater than 150.degree. C. for removal of hydrogen. While this method is effective, the high temperature, three-step purification process requires a relatively expensive purifier.
Another well known method used to purify inert gases uses three different purification beds. The first bed contains palladium or platinum on an alumina/silica substrate. As the unpurified inert gas passes over this catalyst bed at a temperature of at least 350.degree. C., hydrogen combines with added oxygen to form water, methane combines with added oxygen to form carbon dioxide and water, and carbon monoxide combines with added oxygen to form carbon dioxide. The second stage of the purifier uses nickel on an alumina/silica substrate. Once the catalyst is activated, it operates at room temperature allowing the following reactions to occur: nickel combines with carbon monoxide to form nickel carbon monoxide, nickel combines with added oxygen to form nickel oxide, and nickel oxide combines with hydrogen to form elemental nickel and water. The third stage of the purifier is a molecular sieve absorption bed, in which water and carbon dioxide are reversibly absorbed. This stage is operated at room temperature with regeneration at 350.degree. C. Purity in the less than 1 ppb range is achieved with this method. There are two disadvantages with this type of purifier. The first is that oxygen is required, which necessitates introduction of oxygen into the gas stream. The second disadvantage is that the catalyst must be maintained at a temperature of at least 350.degree. C. for these reactions to occur.
A third method for purifying an inert gas stream uses a one-step process. U.S. Pat. No. 4,713,224 teaches a method whereby an inert gas is passed over a catalyst of nickel and an inert material. The catalyst is at least 5% by weight nickel and the method is operated at temperatures of between 0.degree. and 50.degree. C. This method removes carbon monoxide, carbon dioxide, oxygen, hydrogen, and water vapor to levels on the order of 0.1-1.0 ppm, and does not remove methane and other hydrocarbons.